1. Field of the Invention
The present invention relates to an optical system for a see-through head mounted display (HMD), and more particularly, to an optical system for a see-through HMD which may enable information to be easily obtained even during outdoor activities, enable large screen viewing while remarkably reducing a volume and weight thereof, and remarkably reduce manufacturing costs to lower a price of products.
2. Discussion of Related Art
In general, a head mounted display (HMD) apparatus is an image display apparatus in which a virtual large screen is created at a distance by forming a focal point of an image light generated at a position very close to eyes using a precise optical device so that a user can view an enlarged virtual image. As a method of the HMD apparatus, a closed type HMD method to enable only an image light emitted from a display device to be viewed while preventing the surrounding environment from being viewed, and a see-through HMD method to enable the image light emitted from the display device to be viewed while enabling the surrounding environment to be viewed through a window may be given. FIG. 1 shows an example of an optical system of the see-through HMD according to the related art.
First, the see-through HMD shown in FIG. 1 is disclosed in Korean Patent Registration No. 10-0928226, and includes an optical system of an HMD apparatus which includes a display device 10 that emits image lights, a polarized light separator 11 that reflects only a specific polarized light among lights emitted from a micro display panel, a phase retardation plate 12 that converts a linearly polarized light reflected in the polarized light separator 11 into a circularly polarized light or converts an input circularly polarized light into a linearly polarized light, a semi-transparent concave reflection mirror 13 that enlarges the circularly polarized light having passed through the phase retardation plate 12 to send the enlarged light to the phase retardation plate 12 again, and a light opening and closing switch panel 14 that is attached to an outer surface of the semi-transparent concave reflection mirror 13 so that ambient lights can be opened and closed.
By the above-described configuration, beams of only 50% having characteristics of P or S waves among all image lights generated from the display device 10 are transmitted or reflected in the direction of 90 degrees by the polarized light separator 11 disposed so as to be inclined by 45 degrees relative to the display device 10, and the transmitted or reflected beams reach the phase retardation plate 12. Next, the image light linearly polarized from the phase retardation plate 12 is converted into a circularly polarized light to reach the semi-transparent concave reflection mirror 13. Next, the light having reached the semi-transparent concave reflection mirror 13 is reflected to become a circularly polarized light with an inverse rotation direction, and passes through the phase retardation plate 12 and the polarized light separator 11 again to reach user's eyes, and therefore an image enlarged in the semi-transparent concave reflection mirror 13 can be viewed by a user.
However, according to the above-described related art, an amount of the image light generated from the display device 10 is lost by 50%, respectively, while the image light generated from the display device 10 passes through the polarized light separator 11 and the semi-transparent concave reflection mirror 13, and therefore only 25% of the initial amount of light may be transmitted to both eyes and 75% thereof may be lost during a reflection process. As a result, a natural color of an original image light is difficult to be implemented, and therefore there is a problem that a high-luminance light source has to be separately used in consideration of a magnitude of the amount of light which is lost in order to provide an image with appropriate brightness to user's eyes.
In addition, since the polarized light separator 11 is diagonally positioned in a space in which the image enlarged in the semi-transparent concave reflection mirror 13 reaches both eyes, there is a structural problem in an increase in a field of view (FOV) or an eye box, which is an object of a typical HMD. Thus, the increase in the FOV and the eye box may cause an increase in the entire size and weight of the HMD. As a result, when wearing the HMD, a user may easily feel fatigue due to pressurization over the entire face of the user.